I have analysed the five sources that we have gathered so far and they seem to fall into two separate categories:
A. These represent quotes that only tell us that the year 1919 was assumed to have some improbable significance.
1. Mar 1 Wt 1925.
This seems to be the earliest mention of the year 1919 in Watchtower theology. Having reference to the release from detention of the then Directors of the Watchtower Corporation, this release is superimposed with prophetic significance. However there is no suggestion that Jehovah had actually reached out and, after testing these jokers along with the rest of "Christendom", "anointed" them as his favourites
2. April 15 Wt 1926.
This is the second reference to 1919 in Wt literature we have been able to locate, yet no application is made to being either tested or chosen for the great evengelizing work that the Wt now assumes took place in that year. Instead we are given yet another prophetic symbolism. This is of the Elijah-Elisha operation mentioned in the OT. CTR had stated that his generation was to be the last generation in human history. Now JFR hastily jumped in and justified a delay in the end by suggesting that there were to be TWO last generations.
CTR led a group who evidently represented the OT prophet Elijah, and he, JFR representing Elisha, was the lead the next generation into the promised land.
But still no concept of a testing and choosing by Jehovah who was supposedly at his temple doing this work.
3. Yearbook 1927. Here we have a third reference to 1919 but it only confirms the Elijah-Elisha illusion conjured up the previous year.
B. This class of quotes actually answers the question presented by JWfacts.
1. July 15 1930 page 214.
Here is the earliest notation that 1919 was not just a year of prophetic antitypes, but was a time of divine selection for the work of evangelism. For the first time we are informed that Jehovah came to his "temple" for the purpose of choosing his personal and collective representative here on earth. Having examined "Christendom" and rejecting them because of their sustaining and perpetuating "pagan" beliefs, he rejected them and instead settled on the WTBTS. This is despite the fact that there was no meaningful difference in "pagan" convictions held by the Wt leadership.
2. Aug 1 and 15 Wt 1930 paragraphs 45, 46. This reiterates what was stated a month earlier.
So, to answer the question posed in this thread:
The date is July 15, 1930, in the Wt magazine.